A conventional overcurrent relay has a function for opening a usually-closed contact point of an internal contact mechanism and closing a usually-opened contact point of the internal contact point mechanism through a so-called trip operation such that a load such as a motor is stopped by intercepting a main circuit using an electromagnetic contactor and the like when there occurs abnormality such as a case where an overcurrent flows into the main circuit or a case where a main three phase circuit has a defective phase, a function for restoring the internal contact point mechanism to a stationary state by manually closing the usually-closed contact point of the internal contact mechanism and opening the usually-opened contact point after the trip operation, which is also called a manual reset, and a function for restoring the internal contact point mechanism to a stationary state by automatically closing the usually-closed contact point of the internal contact mechanism and opening the usually-opened contact point after a predetermined period of time elapses, which is also called an automatic reset. In addition, the overcurrent relay has a structure switchable between the automatic reset and the manual reset.
In addition, the conventional overcurrent relay is classified into an external power feeding method for supplying driving power through a separate power line other than a main circuit power line and a self-power feeding method for supplying driving power through the main circuit power line using a current transformer (CT).
In an overcurrent relay employing the self-power feeding method, when current does not flow into a coil installed in an electromagnetic contactor according to a trip operation performed by the overcurrent relay, magnetization of the coil is released and the electromagnetic contactor intercepts an electrical connection from a power source of the main circuit to a load. Thereafter, if an internal electric circuit and a magnetic circuit of the overcurrent relay are not configured in such a manner that the automatic reset is distinguishable from the manual reset, when a predetermined period of time elapses, the overcurrent relay closes a usually-closed contact point of an internal contact point mechanism and opens a usually-opened contact point of the internal contact point mechanism automatically, even when the manual reset is set, in the same manner as when the automatic reset is set.
Accordingly, when the manual reset is set, a mechanical configuration is required to prevent the above-mentioned operation. As an example, Japanese Patent Application Publication No. 2001-520795 discloses a trip mechanism allowing switching between the manual reset and the automatic reset. As described in this publication, the trip mechanism allowing switching between the manual reset and the automatic reset prevents the operation of automatically closing a usually-closed contact point of an internal contact point mechanism and opening a usually-opened contact point of the internal contact point mechanism electrically and magnetically, even when the manual reset is set, and allows a reset operation to be performed only when the manual reset is set.
Patent Document 1: PCT Japanese Translation Patent Publication No. 2001-520795